fix: decouple session resume from isFirstVisit guard

When frontmatter validation fails, the step is never written to CAS, so
isFirstVisit remains true on the next run.  Both agent-claude-code and
agent-hermes gated session cache lookup behind !isFirstVisit, which
caused them to start a fresh session (and a new worktree) instead of
resuming the one that already has all the work done.

Changes:
- Remove the isFirstVisit guard from both adapters so they always check
  the session cache.
- When isFirstVisit + cache hit (frontmatter-only failure), send a
  minimal correction prompt via buildFrontmatterRetryPrompt() instead
  of re-sending the full initial prompt — the session already has full
  context, we just need the agent to re-output correctly formatted
  frontmatter.
- Add buildFrontmatterRetryPrompt to util-agent with tests.

Fixes #139

packages/agent-claude-code/src/claude-code.ts

@@ -6,6 +6,7 @@ import {
   type AgentContext,
   type AgentRunResult,
   buildContinuationPrompt,
+  buildFrontmatterRetryPrompt,
   buildRolePrompt,
   buildThreadProgress,
   createAgent,
@@ -189,13 +190,20 @@ async function runClaudeCode(ctx: AgentContext, model: string | null): Promise<A
       ctx.storageRoot,
     );
     if (cachedSessionId !== null) {
+      // isFirstVisit + cache hit = previous run completed but frontmatter
+      // validation failed.  The session already has full context — send a
+      // minimal correction prompt instead of the full initial prompt.
+      const resumePrompt = ctx.isFirstVisit
+        ? buildFrontmatterRetryPrompt(ctx.outputFormatInstruction)
+        : fullPrompt;
+
       try {
         const { stdout, stderr, exitCode } = await spawnClaudeResume(
           cachedSessionId,
-          fullPrompt,
+          resumePrompt,
           model,
         );
-        const result = await processClaudeOutput(stdout, stderr, exitCode, ctx.store, fullPrompt);
+        const result = await processClaudeOutput(stdout, stderr, exitCode, ctx.store, resumePrompt);
         if (result.sessionId !== undefined && result.sessionId !== "") {
           await setCachedSessionId(
             "claude-code",

packages/agent-hermes/src/hermes.ts

@@ -5,6 +5,7 @@ import {
   type AgentContext,
   type AgentRunResult,
   buildContinuationPrompt,
+  buildFrontmatterRetryPrompt,
   buildRolePrompt,
   buildThreadProgress,
   createAgent,
@@ -102,6 +103,8 @@ async function storePromptResult(store: Store, sessionId: string): Promise<{ det
 type PromptAttempt = {
   useContinuation: boolean;
   resumed: boolean;
+  /** True when resuming after a frontmatter-only failure (isFirstVisit + cache hit). */
+  frontmatterRetry: boolean;
 };
 
 async function prepareSession(
@@ -112,7 +115,7 @@ async function prepareSession(
 ): Promise<PromptAttempt> {
   if (resumeDisabled) {
     await client.connect(cwd);
-    return { useContinuation: false, resumed: false };
+    return { useContinuation: false, resumed: false, frontmatterRetry: false };
   }
 
   // Check session cache regardless of isFirstVisit.  A previous run may
@@ -123,19 +126,23 @@ async function prepareSession(
   if (cachedSessionId === null) {
     log("6RWK3N8Q", `no cached session for ${ctx.threadId}:${ctx.role}, starting new session`);
     await client.connect(cwd);
-    return { useContinuation: false, resumed: false };
+    return { useContinuation: false, resumed: false, frontmatterRetry: false };
   }
 
   try {
     await client.resume(cachedSessionId, cwd);
     log("9MHT4V2P", `resumed hermes session ${cachedSessionId} for ${ctx.threadId}:${ctx.role}`);
-    return { useContinuation: true, resumed: true };
+    return {
+      useContinuation: !ctx.isFirstVisit,
+      resumed: true,
+      frontmatterRetry: ctx.isFirstVisit,
+    };
   } catch (error) {
     const message = error instanceof Error ? error.message : String(error);
     log("3XPN7K4W", `session resume failed, falling back to new session: ${message}`);
     await client.close();
     await client.connect(cwd);
-    return { useContinuation: false, resumed: false };
+    return { useContinuation: false, resumed: false, frontmatterRetry: false };
   }
 }
 
@@ -158,9 +165,12 @@ export function createHermesAgent(resumeDisabled: boolean): () => Promise<void>
     ctx: AgentContext,
     useContinuation: boolean,
     beforeTurns: TurnsSnapshot,
+    frontmatterRetry: boolean,
   ): Promise<AgentRunResult> {
-    const effectiveCtx = useContinuation ? ctx : { ...ctx, isFirstVisit: true };
-    const fullPrompt = buildHermesPrompt(effectiveCtx);
+    // Frontmatter retry: session has full context, just re-output the format.
+    const fullPrompt = frontmatterRetry
+      ? buildFrontmatterRetryPrompt(ctx.outputFormatInstruction)
+      : buildHermesPrompt(useContinuation ? ctx : { ...ctx, isFirstVisit: true });
     const startMs = Date.now();
     const { text, sessionId, usage: acpUsage } = await client.prompt(fullPrompt);
     const durationSec = (Date.now() - startMs) / 1000;
@@ -192,7 +202,7 @@ export function createHermesAgent(resumeDisabled: boolean): () => Promise<void>
     const beforeTurns = snapshotTurns(beforeSession);
 
     try {
-      return await runPrompt(ctx, attempt.useContinuation, beforeTurns);
+      return await runPrompt(ctx, attempt.useContinuation, beforeTurns, attempt.frontmatterRetry);
     } catch (error) {
       if (!attempt.resumed) {
         throw error;
@@ -203,7 +213,7 @@ export function createHermesAgent(resumeDisabled: boolean): () => Promise<void>
       await client.close();
       await client.connect(cwd);
       // Fresh session after retry — reset snapshot to zero
-      return runPrompt(ctx, false, ZERO_TURNS);
+      return runPrompt(ctx, false, ZERO_TURNS, false);
     }
   }
 

packages/util-agent/__tests__/frontmatter-retry-prompt.test.ts

@@ -0,0 +1,23 @@
+import { describe, expect, test } from "vitest";
+import { buildFrontmatterRetryPrompt } from "../src/frontmatter-retry-prompt.js";
+
+describe("buildFrontmatterRetryPrompt", () => {
+  test("includes correction instruction", () => {
+    const result = buildFrontmatterRetryPrompt("Use YAML frontmatter");
+    expect(result).toContain("previous run completed");
+    expect(result).toContain("do NOT need to redo any work");
+    expect(result).toContain("corrected YAML frontmatter");
+  });
+
+  test("includes outputFormatInstruction when provided", () => {
+    const instruction = "---\nstatus: $done | $review\nsummary: string\n---";
+    const result = buildFrontmatterRetryPrompt(instruction);
+    expect(result).toContain(instruction);
+  });
+
+  test("works with empty outputFormatInstruction", () => {
+    const result = buildFrontmatterRetryPrompt("");
+    expect(result).not.toContain("\n\n\n");
+    expect(result).toContain("corrected YAML frontmatter");
+  });
+});

packages/util-agent/src/frontmatter-retry-prompt.ts

@@ -0,0 +1,21 @@
+/**
+ * Build a minimal prompt for retrying frontmatter output on a resumed session.
+ *
+ * Used when a previous run completed successfully but frontmatter validation
+ * failed — the session already has full context, we just need the agent to
+ * re-output correctly formatted frontmatter without redoing any work.
+ */
+export function buildFrontmatterRetryPrompt(outputFormatInstruction: string): string {
+  const parts: string[] = [
+    "Your previous run completed all work successfully, but the output format was incorrect.",
+    "You do NOT need to redo any work — all changes are already in place.",
+    "",
+  ];
+  if (outputFormatInstruction !== "") {
+    parts.push(outputFormatInstruction, "");
+  }
+  parts.push(
+    "Please output ONLY the corrected YAML frontmatter block (--- delimited) followed by a brief summary of the work you completed.",
+  );
+  return parts.join("\n");
+}

packages/util-agent/src/index.ts

@@ -12,6 +12,7 @@ export {
 } from "./extract.js";
 export type { FrontmatterFastPathResult } from "./frontmatter.js";
 export { tryFrontmatterFastPath } from "./frontmatter.js";
+export { buildFrontmatterRetryPrompt } from "./frontmatter-retry-prompt.js";
 export { createAgent, parseArgv } from "./run.js";
 export { getCachedSessionId, getCachePath, setCachedSessionId } from "./session-cache.js";
 export { getConfigPath, getEnvPath, loadWorkflowConfig, resolveStorageRoot } from "./storage.js";